"unusual double-lobed comet nucleus."Surely nobody knows whether this is unusual or not. Have we ever got close enough to other comets to see? They are the tiniest specks we've ever explored, the farthest from being 'gravitationally rounded'. They might come triple- or quadruple-lobed, as far as we know, or weirder, fantastic shapes. But I think I know what he means: "unusual" for anything else we've seen OR explored.

67P is the sixth comet we've had a close look at (the previous ones from flyby missions). Emily Lackdawalla put together a scale montage. The most common shape seems to be like a bowling pin; maybe that's related to the distinct double lobes of 67P? Halley almost looks as double from certain angles.

tomatoherd wrote:"unusual double-lobed comet nucleus."Surely nobody knows whether this is unusual or not. Have we ever got close enough to other comets to see? They are the tiniest specks we've ever explored, the farthest from being 'gravitationally rounded'. They might come triple- or quadruple-lobed, as far as we know, or weirder, fantastic shapes. But I think I know what he means: "unusual" for anything else we've seen OR explored.

We've looked at a number with radar. Oblong is common, but not to this degree.

captainwiggins48 wrote:Is it possible that 67/P is not a comet at all. The amount of dust and gas escaping doesn't seem too significant, especially during perihelion, nor does there appear to be much ice on it.

Comets vary widely in activity, and we now believe that many of them are more rock than ice. The orbit, behavior, and material makeup of 67P leave little doubt that it's a comet (as opposed to an odd asteroid, for instance).

Surely nobody knows whether this is unusual or not. Have we ever got close enough to other comets to see? They are the tiniest specks we've ever explored, the farthest from being 'gravitationally rounded'. They might come triple- or quadruple-lobed, as far as we know, or weirder, fantastic shapes. But I think I know what he means: "unusual" for anything else we've seen OR explored.

We've looked at a number with radar. Oblong is common, but not to this degree.

The problem with radar "images" is that they are simply 2D maps of R vs. dR/dt (i.e., radar distance vs. radar velocity). Concentric cylinders around the rotation axis intersect a variety of terrain which are compressed together into reduced radar R vs. dR/dt "image" circles. An a priori radar "image" of Comet 67P would probably look quite different than an actual Rosetta image.

NGC3314 wrote:Emily Lackdawalla put together a scale montage. The most common shape seems to be like a bowling pin; maybe that's related to the distinct double lobes of 67P? Halley almost looks as double from certain angles.

glj wrote:Are the white specks that are visible in the picture background stars or other bits of matter that the comet has in tow in addition to Rosetta?

Some of the "white specks" appear more like streaks, so I would interpret them as trails of dust. Rosetta investigated the comet's "corona" and imaged individual "boulders" with a size of about half a meter orbiting the comet. Some of the "white specks" are in front of the comet, ruling out stars (although they could be bad pixels of the camera). Also, my guess is that stars are usually to faint to be seen on images of astronomical objects lit by the sun (you almost never see them on images of planets, or the ISS), but I don't know the exposure time of this image.

glj wrote:Are the white specks that are visible in the picture background stars or other bits of matter that the comet has in tow in addition to Rosetta?

Some of the "white specks" appear more like streaks, so I would interpret them as trails of dust. Rosetta investigated the comet's "corona" and imaged individual "boulders" with a size of about half a meter orbiting the comet. Some of the "white specks" are in front of the comet, ruling out stars (although they could be bad pixels of the camera). Also, my guess is that stars are usually to faint to be seen on images of astronomical objects lit by the sun (you almost never see them on images of planets, or the ISS), but I don't know the exposure time of this image.

One of the people here at this forum (I forgot exactly who... if you're reading this, I am sorry!) figured out that a surprising number of the dots can also be stars. So it's a mix of stars, CCD artifacts, and likely bits of comet, too.

As a 44 years old lay person, generally interested in space observation and exploration, the Rosetta mission surpasses all the others I have been privileged to witness is my lifetime. The complex and weirdly familiar comet landscape has been a thorough revelation. With O2 having been measured as a significant fraction of the comet atmosphere I wonder if we can really rule out in advance the presence of life.

ygmarchi wrote:As a 44 years old lay person, generally interested in space observation and exploration, the Rosetta mission surpasses all the others I have been privileged to witness is my lifetime. The complex and weirdly familiar comet landscape has been a thorough revelation. With O2 having been measured as a significant fraction of the comet atmosphere I wonder if we can really rule out in advance the presence of life.

Regarding life as we recognize it, free oxygen is not a requirement, but liquid water is (and we don't find persistent liquid water on comets).

ygmarchi wrote:The complex and weirdly familiar comet landscape has been a thorough revelation. With O2 having been measured as a significant fraction of the comet atmosphere I wonder if we can really rule out in advance the presence of life.

Regarding life as we recognize it, free oxygen is not a requirement, but liquid water is (and we don't find persistent liquid water on comets).

https://en.wikipedia.org/wiki/Comet#Com ... ct_on_life wrote:<<The detection of organic molecules, including polycyclic aromatic hydrocarbons, in significant quantities in comets has led some to speculate that comets or meteorites may have brought the precursors of life—or even life itself—to Earth. In 2013 it was suggested that impacts between rocky and icy surfaces, such as comets, had the potential to create the amino acids that make up proteins through shock synthesis. In 2015, scientists found significant amounts of molecular oxygen in outgassings from comet 67P, an indicator that presence of that molecule may occur naturally more often than it had been thought, and thus that it may not be as strong an indicator of alien life as has been supposed.>>

<<The Great Oxygenation Event (GOE), also called the Oxygen Catastrophe, Oxygen Crisis, Oxygen Holocaust, Oxygen Revolution, or Great Oxidation, was the biologically induced appearance of dioxygen (O2) in Earth's atmosphere. Geological, isotopic, and chemical evidence suggest that this major environmental change happened around 2.3 billion years ago.

Cyanobacteria, which appeared about 200 million years before the GOE, began producing oxygen by photosynthesis. Before the GOE, any free oxygen they produced was chemically captured by dissolved iron or organic matter. The GOE was the point when these oxygen sinks became saturated and could not capture all of the oxygen that was produced by cyanobacterial photosynthesis. After the GOE, the excess free oxygen started to accumulate in the atmosphere.

Free oxygen is toxic to obligate anaerobic organisms, and the rising concentrations may have wiped out most of the Earth's anaerobic inhabitants at the time. Cyanobacteria were therefore responsible for one of the most significant extinction events in Earth's history.But research has shown that microbial mats of oxygen-producing microbes will produce a thin layer, one or two millimeters thick, of oxygenated water in an otherwise anoxic environment even under thick ice, and before oxygen started accumulating in the atmosphere, organisms living on these mats would already be adapted to being exposed to oxygen. Additionally, the free oxygen reacted with atmospheric methane, a greenhouse gas, greatly reducing its concentration and triggering the Huronian glaciation, possibly the longest snowball Earth episode in the Earth's history.

Eventually, aerobic organisms began to evolve, consuming oxygen and bringing about an equilibrium in its availability.>>